在VScode maven项目中创建servlet

Question

firstServlet.java的代码-

import javax.servlet.http.HttpServlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.*;
import javax.servlet.http.*;

public class FirstServlet extends HttpServlet{
    public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException  {
        
        System.out.println("this is get method....");
        res.setContentType("text/html");
        PrintWriter out = res.getWriter();
        out.println("<h1> this is get method of servlet</h2>");
        
        
    }

}

<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
  <display-name>Archetype Created Web Application</display-name>

  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>


  <servlet>
    <servlet-name>first</servlet-name>
    <servlet-class>FirstServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>first</servlet-name>
    <url-pattern>/first</url-pattern>
  </servlet-mapping>

</web-app>

在这个文件中，web-app标记显示了一条带有错误消息的红线-web-app元素是web应用程序的部署描述符的根。

来源:web-app_2_3.dtd

元素类型“web-app”的内容必须与“（icon？,display-name？,description？,distributable？,context-param*,filter*,filter-mapping*,listener*,servlet*,servlet-mapping*,session-config？,mime-mapping*,welcomple-file-list？,error-page*,taglib*,resource-env-ref*,resource-ref*,security-constraint*,login-config？,security-role*,env-entry*